In general, a coated paper is manufactured by coating a paper with an inorganic pigment such as clay, calcium carbonate, aluminum hydroxide (Al(OH)3) or titanium dioxide (TiO2), in which a natural binder such as casein or starch, or an artificial binder such as styrene-butadiene latex, polyvinyl alcohol or acryl latex is used with other additives such as a dispersive agent, thickener or insolublizer. In manufacturing the coated paper, the most important component is an inorganic pigment and binder, which must be selected to produce balanced physical properties.
Clay and calcium carbonate are the most frequently used inorganic pigments. Clay of a disk-like structure has an advantage of resulting in a high paper gloss and print gloss, while it has a low fluidity and high binder requirement. Calcium carbonate results in a good fluidity, adhesiveness, ink-acceptance, paper brightness and opacity, while it has a problem that the coating solution requires a high chemical stability against calcium cation.
Recently, in order to improve productivity and to reduce the energy for drying after coating, the concentration of solid components has increased in the coating solution. In this regard, however, the viscosity of the paper coating composition increases and the fluidity of the composition decreases, thereby reducing the operation efficiency.
As the manufacturing of papers speeds up, it is proceeded to increase the coating rate for improving the productivity and managing the increasing demands for printing material. A recent coating rate reaches such a high level as 1,000˜1,500 m/min. As a coating rate increases, the shearing stress at coating also increases, therefore, the fluidity of high shear becomes essential to coating. In this place, “high shear” means the shear rate of above several thousands sec−1.
The fluidity of low shear affects the transportation of coating solution and coating process, in which “low shear” means in general the shear rate of below several hundreds sec−1.
As described above, in order to increase the concentration of coating solution and coating rate, the problem of low-shear/high-shear fluidity of coating solution should be solved previously.
As a measure for the problem, it is disclosed that water-soluble natural binders with high thickening effect, such as starch or casein, have been replaced with artificial binders or the ratio of heavy calcium carbonate of fine particles with a good fluidity has been increased as a pigment. In recent practices, the ratio of calcium carbonate has increased instead of clay. For all the merits of the above method, however, it has some defects that the physical properties of coating surface decrease in paper gloss, print gloss and smoothness, and the chemical and mechanical stability of coating solution lowers.
Further, it is also described that the low-shear/high-shear fluidity increases by using latex of high fluidity. Since the fluidity of coating solution depends actually on latex, this plan is considered to be the most practical and safest solution to improve the fluidity of coating solution. Accordingly, the requirement of latex having excellent fluidity is increasing in the paper manufacturing industry.
In case the particle diameter of latex decreases, the high shear viscosity favorably lowers in general. In this case, however, low shear viscosity disadvantageously increases. Accordingly, desired fluidity cannot be obtained only by adjusting the particle diameter of latex, and it is required to treat properly the surface of latex.